1. Field of the Invention
The present invention relates to a polygonal-line characteristic amplifier and, more particularly, to a polygonal-line characteristic amplifier having characteristics that its gain becomes high when an input voltage is low, while it becomes low when the input voltage is high.
2. Description of the Related Art
As a buffer amplifier, conventionally used is the so-called voltage-follower circuit, as shown in FIG. 1, in which an operational amplifier 2 has an inverting input terminal (-) connected to an output terminal 3 thereof so as to form an output of the voltage-follower circuit, and a non-inverting input terminal (+) serves as a signal input terminal 1. An input/output gain of the voltage-follower circuit shown in FIG. 1 is "1", so that the input-output characteristic at this time between an input voltage V.sub.IN and an output voltage V.sub.OUT is as shown in FIG. 2 in which an output signal having the same voltage as that of an input signal applied to the input terminal 1 appears at the output terminal 3, while the input-output characteristic becomes saturated at a certain point in an output dynamic range. This is because the input-output characteristic depends on a positive power supply voltage.
The polygonal-line characteristic amplifier using the conventional voltage-follower buffer amplifier has a disadvantage in that the output voltage of the circuit is limited to the output dynamic range of the amplifier, so that the output voltage becomes fixed to a constant value which is limited by the maximum output voltage of the amplifier when the level of the input signal becomes equal to or more than a certain value.